C# WebClient.DownloadString() Returns String With Perculiar Characters?

I have an issue with some content that we are downloading from the web for a screen scraping tool that I am building.in the code below, the string returned from the web client download string method returns some odd characters for the source download for a few (not all) web sites.I have recently added http headers as below. Previously the same code was called without the headers to the same effect. I have not tried variations on the 'Accept-Charset' header, I don't know much about text encoding other than the basics.The charachters, or character sequences that I refer to are:

""
and
"Â"

These characters are not seen when you use "view source" in a web browser. What could be causing this and how can I rectify the problem?

string urlData = String.Empty;
WebClient wc = new WebClient();
// Add headers to impersonate a web browser. Some web sites
// will not respond correctly without these headers
wc.Headers.Add("User-Agent", "Mozilla/5.0 (Windows; U; Windows NT 6.1; en-GB; rv:1.9.2.12) Gecko/20101026 Firefox/3.6.12");
wc.Headers.Add("Accept", "*/*");
wc.Headers.Add("Accept-Language", "en-gb,en;q=0.5");
wc.Headers.Add("Accept-Charset", "ISO-8859-1,utf-8;q=0.7,*;q=0.7");
urlData = wc.DownloadString(uri);